1. Field of the Invention
The present invention relates to a dual-frequency laser interferometer, used for example to measure distance.
2. Description of Related Art
One such interferometer (see e.g. EP 194941) comprises a laser and an acousto-optic modulator which produce a pair of beams, orthogonally polarised with respect to each other, and having a difference in frequency of .DELTA..omega.. The two beams, by virtue of their different polarisations, are diverted down separate arms of a Michelson-type interferometer. The beam reflected from the measurement arm undergoes a frequency shift due to the Doppler effect when the retro-reflector of the measurement arm moves. By comparing the beat frequencies between the two beams before and after reflection from the two arms of the interferometer, the magnitude of movement of the measurement arm may be determined. The beat frequency of the two beams after reflection is determined by interfering the beams and detecting the intensity of the resultant interference beam with a photo-detector.
A problem with the apparatus described above is that the beat frequency of the two beams before reflection (i.e. the "reference beat frequency") is derived directly from an oscillator used to drive the acousto-optic modulator. The apparatus is thus highly sensitive to a change in the propogation time of the signal from oscillator, or the propogation time of the light beams from the acousto-optic device through the interferometer and the electrical signal from the photo-detector. A change in one of the propogation times not occurring in the other will cause a relative phase shift in the two signals and a consequent error in the value of the measurement. Such a circumstance may readily arise due to a change in temperature in one part of the apparatus, which effects the propogation time of one of the signals, and which is not experienced in the other part of the apparatus. For example, it has been found that a temperature change of approximately 1.degree. C. can cause a measurement error of about 0.3 microns. This is obviously undesirable in an apparatus which is intended to be a precision measurement apparatus.
It is also known (e.g. from U.S. Pat. No. 3,534,292) to provide a Zeeman-split laser which produced a pair of frequency-shifted orthogonally polarised beams. The beat frequency between the two beams is derived by interfering the beams, and detecting the intensity of the resultant interference beam with a photo-detector.